The present invention relates to the field of digital image processing and, more particularly, to the processing of digital images which are displayed on a screen, in particular a computer or television screen.
At present, the portable computer and television markets are beginning to overlap due to the development of the Internet and of interactive television. It is therefore important to be able to display graphical images (such as the web pages broadcast on the Internet) on a standard television screen in a visual format that is acceptable to a user.
Prior art systems used to suppress or limit a flicker effect caused by strong contrasts of the graphical images are often referred to as flicker filters. Since the flicker effect is due to the contrast between two consecutive lines (i.e., to high frequencies on a vertical column of the image), typical anti-flicker filter systems implement a low-pass vertical digital filtering. After the filtering, two consecutive lines are closer together in terms of color and the flicker effect is diminished.
Moreover, graphical objects are often displayed on the screen. These objects may take the form of windows or sub-windows whose height and/or width the user may wish to modify. The processing required by a change of dimensions of the graphical objects displayed involves storing pixels from one or more lines in fast memories, resulting in high costs.
Various image formats may be used. In the YCbCr 420 format, which is known in the art, luminance data and chrominance data may be stored separately. The chrominance data are four times less numerous than the luminance data, twice in the horizontal direction and twice in the vertical direction. A particular chrominance data item relates to a point situated at the center of four neighboring points for which luminance data are available, and the various points are coplanar. The YCbCr 420 format is economical in terms of memory space used on account of the small number of chrominance data, but is not suitable for processing graphical objects.
The YCbCr 444 format is also known in the art. In this format, the luminance data and chrominance data are equal in number and relate to points with the same coordinates. It is a known practice to prepare an image for display in the YCbCr 444 format by concatenating the luminance and chrominance data one after another for each point of the image.
It is possible to carry out an image format transformation to go from the YCbCr 420 format to the YCbCr 444 format in real time in a display facility immediately before a display. Even so, this technique is not suitable for processing graphical objects.
An object of the present invention is to display graphical images (such as web pages broadcast on the Internet) on a standard television screen in a visual format that is acceptable to a user.
Another object of the invention is to provide a method of economical graphical transformation which does not require an excessive amount of memory and may be used for images of high quality regardless of the size of the graphical object processed.
Yet another object of the present invention is to provide a method of graphical transformation capable of performing changes of format of an image to render the image suitable for digital graphical processing and to do so relatively simply.
According to the invention, a transformation method for digital images including luminance data relating to points with first coordinates and chrominance data relating to points with second coordinates where one of the luminance data and the chrominance data is more numerous than the other includes processing the least numerous data by filtering to obtain least numerous data relating to points with identical coordinates to those of the most numerous data. Moreover, a vertical filtering and a horizontal filtering may be performed, and an initial phase shift filtering may also be performed.
Furthermore, the number of the least numerous data may be increased in the horizontal direction or in the vertical direction. The luminance data may be the most numerous and the chrominance data may be the least numerous, for example. Specifically, the luminance data may be four times more numerous than the chrominance data. The luminance and chrominance data may also be stored in a memory and processed to change dimensions of the image. Additionally, the change of dimensions may be performed by hardware used for the transformation.
A graphical processing system according to the present invention capable of implementing the above method as well as other graphical processing operations is also provided. Such other graphical processing operations may include, for example, changing dimensions of a graphical object to obtain an enlargement or a reduction of a window displayed on a television or computer screen. The system may include a first memory and a second memory for storing pixels awaiting processing, a graphical processing system for changing the dimensions of the images arranged downstream from the second memory, and an arithmetic logic unit arranged downstream from the first memory and from the graphical processing system for arranging data received therefrom in the order desired for subsequent display. The graphical processing system may be used to process the least numerous data to obtain least numerous data relating to points with identical coordinates to those of the most numerous data.
The system may be used for graphics subsystems including a two-dimensional accelerator which acts as an operator on rectangular graphical objects, such as windows. This accelerator or module for transferring bit blocks, referred to as a xe2x80x9cblitter,xe2x80x9d is used for updating zones of the image, and it may also advantageously be used in format transformation and in other graphical processing operations.
The invention therefore allows a change of image format carried out time relative to the display, i.e., from memory to memory. It is possible to accept a relatively slow speed of execution of the change of format relative to the speed which would be demanded in real time. Hence, it is possible to have quality processing implementing one or more filters with a considerable number of intervals (e.g., five).
The passband requirements of the display remain unchanged. Thus, since the processing is from memory to memory, the entire device for generating digital images is not forced to follow the standard order of refreshing of the display screen. The amount of memory required for the line buffer memories can therefore be reduced significantly. The filter need not be duplicated in the case of a global system linked to several graphical displays.
YCbCr 420 or 444 does not specify that the luminance and the chrominance must be stored in separate memories. Yet, the present invention is applicable when one seeks to access YCbCr 420-coded pixels stored in two separate memories.